4. Pachymetry Central corneal thickness is measured with ultrasound pachymetry. The right eye is tested first. A drop of 0.5% proparacaine is instilled for anesthesia. The patient is asked to look straight ahead at a distant object or fixation target. The pachymeter probe is lined up with the center of the pupil and slowly advanced until it contacts the cornea. The probe is withdrawn when an audible signal is made indicating that a measurement has been recorded. The patient is instructed to blink. The procedure is repeated to obtain three separate readings, and the investigator records the measurements. After testing of the right eye is complete, the same technique is applied to testing of the left eye.